Portable back rest



FIG. 1 is a side elevational view of a portable back rest showing my design, the opposite side being a mirror image thereof;

FIG. 2 is a top view;

FIG. 3 is a bottom view;

FIG. 4 is a front view; and

FIG. 5 is a rear view. 

The ornamental design for a portable back rest, as shown and described. 